The main use of the curb chain is to control the leaver action of a curb bit.

Additionally, it also helps to keep the bit steady and in place within the mouth, and helps to hold a lip strap in place.The curb chain applies pressure to the curb groove under a horse’s chin when the curb rein of the bit is used. When the curb rein is pulled, the shank of the bit rotates back towards the chest of the horse and the cheek  of the bit rotates forward . The curb chain is attached to the hooks at the end of the cheek, so, as the cheek moves forward, the chain is pulled and tightened in the curb groove. Once it comes in contact with the curb groove of the horse it acts as a fulcrum   ( fulcrum is the support about which a leaver pivots), causing the cannons of the bit mouthpiece to push down onto the horse’s bars, thus amplifying the bit’s pressure on the bars of the horse’s mouth.

The tightness of the curb chain therefore has a great effect on the action of the bit. If the bit is used without a curb chain, it loses its leverage action. If used with a loose curb chain, it allows the shanks to rotate more before the curb chain is tight enough to act as a fulcrum and exert pressure. This extra rotation can warn the horse before pressure is exerted on his mouth, so he may respond beforehand. However, a very loose curb chain can be undesirable, especially if the port is high. It allows the bit to rotate in the mouth too much, causing the port to become more vertical and press against the palate, which is painful and can damage the mouth, and can cause the horse to gape. Additionally, it can completely nullify the correct action of the curb, making its use pointless.

There are two undesirable consequences when using a very tight curb chain. First, the bit immediately exerts curb pressure and increased pressure on the bars as soon as pressure is applied to the reins. Therefore, a tight curb chain is much harsher, and provides less finesse in signaling the horse than a looser curb chain would, as the horse is never given a chance to respond before the bit pushes into his bars. Secondly, a very tight curb chain causes the mouthpiece to constantly push down on the sensitive tongue, never allowing the horse relief. In severe cases, the tongue can lose blood supply.